The present invention relates generally to the field of spin-dependent scattering of electrons to spin polarize current. In particular, the present invention relates to an artificial magnet with spin polarization enhancement for use in magnetoresistive devices.
Magnetoresistive (MR) devices generally operate by responding to changes in local magnetic flux. For example in a magnetic data storage and retrieval system, a magnetic recording head typically includes a reader portion having a magnetoresistive (MR) sensor for retrieving magnetically encoded information stored on a magnetic disc. Magnetic flux from the surface of the disc causes rotation of the magnetization vector of a sensing layer or layers of the MR sensor, which in turn causes a change in electrical resistivity of the MR sensor. The sensing layers are often called free layers, since the magnetization vectors of the sensing layers are free to rotate in response to external magnetic flux. The change in resistivity of the MR sensor can be detected by passing a current through the MR sensor and measuring a voltage across the MR sensor. External circuitry then converts the voltage information into an appropriate format and manipulates that information as necessary to recover the information encoded on the disc.
For all types of MR sensors, magnetization rotation occurs in response to magnetic flux from the disc or other magnetic media. As the recording density of magnetic discs continues to increase, the width of the tracks on the disc must decrease, which necessitates more sensitive sensor devices in order to supply the necessary signal amplitude to the preamplifier within a hard disc drive.
MR sensors of present interest can be characterized in two general categories: (1) giant magnetoresistive (GMR) sensors, including spin valve sensors, and (2) tunneling magnetoresistive (TMR) sensors.
GMR sensors have a series of alternating magnetic and nonmagnetic layers. The resistance of GMR sensors varies as a function of the spin-dependent transmission of the conduction electrons between the magnetic layers separated by the nonmagnetic layer and the accompanying spin-dependent scattering which takes place at the interface of the magnetic and nonmagnetic layers and within the magnetic layers. The resistance of a GMR sensor depends on the relative orientations of the magnetization in consecutive magnetic layers, and varies as the cosine of the angle between the magnetization vectors of consecutive magnetic layers.
Conventional GMR sensors include many classes of sensors including current-in-plane spin valve (CIP-SV), CIP SAF-only SV (synthetical antiferromagnetic), CIP trilayer SV, and current-perpendicular-to-plane (CPP)-SV sensors. A limiting factor in many of CIP sensors is the decrease in amplitude due to loss of free layer area as the reader width decreases. Conventional GMR sensors rely primarily on positive spin symmetry effects. Therefore, further improvement in spin-dependent scattering is one method to improve amplitude in these devices.
TMR sensors have a configuration similar to GMR sensors, except that the magnetic elements (also referred to as electrodes) of the sensor are separated by a barrier layer that is thin enough to allow electron tunneling between the magnetic elements. A first magnetic element serves as a spin polarized source, while the second magnetic element serves as a spin detector or drain. The tunneling probability of an electron incident on the barrier from one electrode depends on the spin state of the electron and the relative orientation between the magnetization of the spin polarized source and the spin detector.
For a TMR device, the magnetoresistance (MR), used as a measure of device sensitivity, is equal to 2P1P2/(1−P1P2) where P1 is the spin polarization for the spin polarized source and P2 is the spin polarization for the spin detector. Spin polarization is defined as (N↑−N↓)/(N↑+N↓), where N↑, N↓ are the number of spin-up and spin-down electrons respectively. Therefore in TMR devices, the more effective a spin polarized source is at providing spin selected electrons (spin polarized sense current), the greater the sensitivity of the TMR device. Consequently, there remains a need in the art for improved spin polarization for use in these sensors and other MR devices.